Concerning this problem with nerd fonts that are not rendered anymore: if you explicitly set a nerd font it works. Previously, as long as you had installed a nerd font in your system, even if you hadn’t set that font explicitly, Qt was looking for a font providing the nerd glyph. Now, this behavior has been intentionally removed: https://code.qt.io/cgit/qt/qtbase.git/commit/?id=a44b6950268214d802bc7ce7df09975261263e31.
See also my comment on the other topic: KDE Plasma 6 and nerd fonts - #16 by lorebett